Sleeping bags originally were in the form of a blanket that had two side edges sewn together. As sleeping bag technology advanced, the wool blanket material commonly used became replaced by materials having a higher insulating value and lower weight. The improved sleeping bags were formed by placing an insulating material between two layers of a durable retaining material Down was, and still is, favored for use as the insulating material in sleeping bags. There are presently a number of synthetic insulating materials that are also used such as HOLLOFILL, FIBERFILL THINSULATE and polyester batting. Common retaining materials are cotton, taffeta and ripstop nylon.
As sleeping bag material technology improved, the design of the bag was also improved. Sleeping bags were originally rectangular in shape. It was later learned that tapering the bottom end of the bag improved the bag's ability to keep the user's feet warm. By tapering the bag, the amount of contained dead air space is minimized. This led to a large variety of tapered bags. However, the design still included a large top opening that allowed a significant amount of heat to escape.
The next improvement was a modification in the top of the bag. The mummy style of bag was adopted and its use greatly alleviated the noted heat loss problem. In a mummy bag, the bag closely encases the entire body of the user save for a small opening above the user's face. This type of bag was found to be highly efficient due to both the minimal size of the opening and the bag's close fitting nature. However, inherent in the design of the mummy bag is a low degree of adaptability to fit different length users.
The size of a mummy-type bag must be closely matched to the size of the user. If the bag is overly long, an air space is created below the user's feet. This greatly reduces the bag's efficiency and may lead to insufficient heating of the user's lower extremities. For a fully grown adult, a properly sized bag can be chosen and this particular problem is minimized. For a growing child however, this limitation of the mummy-type bag is a significant problem.
Children have traditionally been relegated to using rectangular bags. These bags, while low in efficiency, do accommodate a growing child since they are not close fitting. Children's bags are typically four to five feet long and are capable of fitting a child of toddler age through early teen-age. Due to the child's rapid growth, use of a mummy-type bag for the child would require the purchase of a longer bag almost yearly. Therefore, most parents opt for a traditional, less efficient rectangular bag for their child until the child's growth slows.